Data storage devices of the type known as "Winchester" or "hard" disc drives are typically provided with a plurality of rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. A controllably positionable actuator is disposed adjacent the discs, the actuator including a plurality of heads which are used during write and read operations to store and retrieve user data from tracks defined on the disc surfaces.
A closed loop servo system is used to control the position of the heads with respect to the tracks on the disc. More particularly, the actuator typically includes a coil of a voice coil motor (VCM) such that currents applied to the coil by the servo system cause the heads to move relative to the tracks in a controlled manner. A read/write channel, responsive to the heads, controls the transfer of user data between the discs and a host computer in which the disc drive is mounted.
As will be recognized, typical disc drives utilize various techniques to minimize the effects of anomalous conditions upon the transfer of the user data between the discs and the host computer. For example, typical read/write channels employ various types of filtering and signal level detection to account for channel noise that would tend to interfere with the reliable transmission of the user data. Further immunity to various anomalous conditions is achieved through the use of error detection and correction codes and associated circuitry for encoding and subsequently decoding the bits transferred to and from the discs. Moreover, typical servo systems use threshold detection and other well known techniques to filter out invalid head position and velocity information in order to prevent unnecessary and time consuming head position corrections.
Nevertheless, anomalous conditions can arise during the life of a disc drive which tend to decrease the overall data transfer performance of the drive. Highly sensitive magneto-resistive (MR) heads, which detect the selective magnetization of tracks through corresponding changes in resistance of MR elements of the heads, are particularly susceptible to anomalous conditions commonly referred to as "thermal asperities". Broadly speaking, a thermal asperity comprises a rapid change in the temperature and/or heat dissipative state of an MR head as a result of the physical interaction between the head and the surface of a disc. Particularly, a contact thermal asperity involves the MR head physically contacting a localized "hill" on the surface of the disc (or a particulate disposed on the disc); the kinetic energy from the impact can cause read channel distortion for several microseconds, sufficient to prevent recovery of up to 100 bytes or more of information on the disc, depending on the transfer rate and filtering characteristics of the channel. A non-contact thermal asperity occurs as the heat dissipative characteristics of a head rapidly change as the head passes over, but does not contact, a localized "hill" or "valley" on the disc surface; while non-contact thermal asperities adversely affect the data transfer capabilities of a disc drive to a lesser extent than do contact thermal asperities, non-contact thermal asperities can still distort the read channel for several hundred nanoseconds, potentially affecting tens of bytes of information. Significantly, thermal asperities can "grow" over time so that typical disc drives can generally be expected to experience increasingly greater amounts of read channel and servo system errors as a result of thermal asperities. Other anomalous conditions stemming from such causes as, for example contamination and wear, have also been found to generally decrease disc drive data transfer characteristics over time.
A read error is typically declared at such time that user data retrieved from a data block, or sector, of a track contains one or more uncorrectable errors. It will be recognized that a typical disc drive includes internally programmed error recovery routines so that, in the face of such an error, the disc drive proceeds to apply a variety of corrective operations in order to recover the user data. The use of such corrective operations will limit the maximum data transfer rate of the disc drive, especially as certain types of corrective operations can require a relatively significant amount of time to complete. Occasionally, having exhausted all available avenues for data recovery without success, the disc drive will declare a hard error and reallocate the data block by mapping out the bad data block and substituting therefor a new, unused data block from a different physical location in the drive.
It will be readily apparent that users increasingly rely on the ability of disc drives to efficiently store and retrieve user data. Thus, persistent read and servo errors not only reduce the effective data transfer rate of a drive, but when sufficiently severe, can unhappily prevent the retrieval of data that was previously stored to the discs of the drive.
Accordingly, it is becoming increasingly important that modem computer systems be provided with efficient and reliable data storage and retrieval characteristics. There is a continuing need, therefore, for improvements in the art whereby the effects of anomalous conditions upon the operational performance of disc drives can be minimized.